In many chemical processes close control of the ratio of reactants is necessary to ensure that the reaction favors formation of the desired product. For example, it is known that a tert-alkyl ether can be prepared by reacting a primary alcohol with an olefin having a double bond on a tertiary carbon atom, as methanol reacts with isobutylene or isoamylene to form respectively methyl tert-butyl ether (MTBE) or tertamyl methyl ether (TAME). The use of excess of methanol, however, renders the purification of ethers very expensive because of the formation of azeotropes, with the resulting difficulties in distillation of reaction effluent. Many etherification processes utilize feedstreams which are produced by some previous process and are often delivered directly to the ether reactor from a process such as a cracker unit or a dehydrogenation unit in the same or a nearby plant. Under such conditions reactive olefin constituents in the olefin feedstream may be present in variable concentrations due to process variations associated with its production in the previous process. Regardless of such variations it is desired to maintain a close ratio of olefin and alcohol reactants flowing to the ether reactor. Control in such a manner is both more in need and more difficult where the reactant component is present in a feedstream in a relatively low concentration and/or subject to wide variations in concentration.
In an etherification process individual feedstreams of reactive olefin and alcohol are provided to a mixer with the mixed stream fed to the reactor. As used herein an individual reactant stream is a stream containing at least one reactant but not all of the reactants required for a desired reaction. Feedstreams containing unreactive hydrocarbons such as isobutane, herein called lean feedstreams, may also be advantageously employed in the etherification process. For example, a lean feed supplied to the reactor along with the individual reactant streams can aid in the separation steps following the reaction. In the past typical control approaches for maintaining a constant concentration ratio of olefin to alcohol in the reactor feedstream relied on independently controlling the flow rate of the olefin containing feedstream to a flow set point. Changes in concentration of reactive olefin in the olefin containing stream, which would cause a variation in the olefin to alcohol concentration ratio in the reactor feed, are detected by analyzing the mixed feedstream to determine the actual ratio of olefin/alcohol, and then manipulating the individual alcohol stream to avoid a change in the ratio. While the above described control method which manipulates the flow of one or more individual feedstreams in response to measured analysis from a mixed feedstream has proven effective for controlling the olefin to alcohol concentration ratio, it is subject to certain limitations. For example, the analysis of a mixture containing alcohol and olefin components is complex and the analyzer is difficult to calibrate. Further the analysis equipment is difficult to maintain because of the alcohol present in the sample.
It is thus an object of this invention to reduce the cost for recovery of unreacted constituents in a reactor effluent stream.
Another object is to continuously control the flow of each reactive component in a mixed feedstream, with fixed ratios between reactive components, without measuring concentration of each reactive component in the mixed stream.
It is a more specific object of this invention to maintain a desired isoolefin to alcohol ratio in a feedstream to an ether reactor.
It is still another object is to improve efficiency in a process for producing a high purity ether product.